Enzymatic Synthesis of Heparin - Sustainable Biotechnology: Sources of Renewable Energy - page 263

N -deacetlatylase/ N -sulfotransferase activities [30-33]. After the N -sulfated back-
bone is generated, the C 5 -epimerase converts the neighboring GlcUA unit on
the reducing side to an IdoUA unit. The chain modification proceeds with 2- O -
sulfation at the IdoUA/GlcUA (with a preference to IdoUA), 6- O -sulfation at the
glucosamine, and 3- O -sulfation at the glucosamine by different O -sulfotransferases
(OSTs [22]). C 5 -epimerase and 2- O -sulfotransferase (2-OST) only have one iso-
form, while 6- O -sulfotransferase (6-OST) has three [34] and 3- O -sulfotransferase
(3-OST) has seven isoforms [20].
2.3 The Role of HS/Heparin in Regulating the Blood Coagulation
Heparin has remained one of the main anticoagulant drugs since it was introduced
in the 1930s [13]. The coagulation cascade consists of a series of proteases and their
precursors. Factox Xa and factor IIa (or named as thrombin) are two key proteases
in controlling the blood coagulation cascade. The advantages of heparin over other
anticoagulant drugs include following: heparin is the only drug can inhibit both fac-
tor Xa and IIa activities, heparin has a fast anticoagulant response, and the excessive
anticoagulant activity can be reversed by protamine [20]. Heparin is an exclusive
product of mast cells, and is released during degranulation of mast cells. Therefore,
HS, rather than heparin, is considered to be the “natural anticoagulant” in humans as
HS is present on the surface of endothelial cells of blood vessels. The anticoagulant
heparin and HS contain a structurally defined AT-binding pentasaccharide sequence
with the structure of -GlcNS(or Ac)6S-GlcUA-GlcNS3S6S-IdoUA2S-GlcNS6S-
(Fig. 3) [6-9]. The AT-binding site is the essential motif for the anticoagulant
activity of heparin and HS. However, pharmaceutical grade heparin and low-
molecular weight heparin (LMWH) also lead to untoward side effects include
bleeding and HIT. HIT occurs in approximately 3% of patients receiving unfrac-
tionated heparin and about 0.2% in those patients receiving low molecular weight
heparin [35].
Fig. 3 The structure of AT-binding pentasaccharide. The 3- O -sulfation of glucosamine residue
(3- O -sulfation is in red) is the critical modification to generate the AT binding site. GlcNS(Ac)6S,
GlcUA, GlcNS3S6S, IdoUA2S and GlcNS6S represent the abbreviation of the individual monosac-
charide residue